Switching power converters include a controller that controls the cycling of a power switch to regulate the delivery of power to a load. Like all electronic circuits, the controller requires a power supply voltage. During normal operation, the generation of the power supply voltage for the controller may be derived from the output power delivery. For example, a flyback converter typically includes an auxiliary winding for its transformer. Power is reflected onto the auxiliary winding while the secondary current flows following the cycling off of the power switch during each power switch cycle. A power supply capacitor couples to the auxiliary winding through a normal operation charging circuit that is active during normal operation to maintain the charging of the power supply capacitor with the power supply voltage. The regulation of the power supply voltage is in a non-isolated switching power converter such as a buck converter is even more convenient in that the power supply capacitor can be coupled to the output voltage during normal operation to charge the capacitor with the power supply voltage.
Although the controller power supply voltage may thus be maintained by siphoning from the power delivered to the load during normal operation, there is no such power delivery at startup. It is thus conventional for a switching power converter to include a separate startup charging circuit that drives the power supply capacitor at startup. For example, a switching power converter that receives its input power from an AC mains will typically include a diode bridge that rectifies the AC input voltage. The resulting rectified input voltage is smoothed through a bulk input capacitor. A startup circuit thus couples to an input node carrying the rectified input voltage stored across the bulk input capacitor to generate the power supply voltage at power-up of the switching power converter. When normal operation ensues, the startup circuit is deactivated so that the normal operation charging circuit may maintain the power supply voltage for the controller. The startup circuit typically uses a relatively-high-impedance current limiting resistor to limit the amount of current into the power supply capacitor at power-up of the switching power converter. Such current limiting is important because the controller receives the controller power supply voltage over a power pin (Vcc pin). Should the Vcc pin have a short to ground, the relatively-high-impedance current limiting resistor prevents excessive charging currents through the startup circuit that could otherwise damage components of the startup circuit. But the high impedance of the current limiting resistor can negatively impact startup operation of the switching power converter. For one example, the relatively-high-impedance of the current limiting resistor lengthens the amount of time until a suitably-high level is reached for the power supply voltage so that normal operation may begin. In addition, the likelihood of a voltage ringing increases as the resistance of the current limiting resistor is increased due to undesirable interactions with the parasitic capacitance of the power switch transistor.
It is thus desirable to reduce the resistance of the current limiting resistor in the startup circuit to reduce the startup charging period and any voltage ringing. But such a reduced impedance for the current limiting resistor then exposes the startup circuit to damage should the Vcc pin of the controller have a short or soft-short to ground. Accordingly, there is a need in the art for startup circuits for switching power converters with reduced startup charging periods without incurring damage from any short circuit conditions on the power pin of the controller.